Imaging Sources in the Third Realization of the International Celestial Reference Frame

TL;DR

This paper reports on a year-long VLBI observational campaign that monitored thousands of quasars to refine the celestial reference frame, focusing on source structure, flux density, and spectral properties for improved astrometric accuracy.

Contribution

It provides new high-precision astrometric data and analysis of source structure for the ICRF3, enhancing the understanding of quasar properties used in celestial reference frames.

Findings

70% of sources are compact at X-band, suitable for ICRF use

89% of sources have flat spectral index, indicating stable emission

Median flux densities are 0.13 Jy at S-band and 0.09 Jy at X-band

Abstract

The third iteration of the International Celestial Reference Frame (ICRF3) is made up of 4536 quasars observed at S/X bands using Very Long baseline Interferometry (VLBI). These sources are high redshift quasars, typically between $1<z<2$, that are believed to host active galactic nuclei (AGN) at their centers. The position of compact radio sources can be determined better than sources with large amounts of extended radio structure. Here we report information on a series of 20 observations from January 2017 through December 2017 which were designed for precise astrometry and to monitor the structure of sources included in the ICRF3. We targeted 3627 sources over the one year campaign and found the median flux density of 2697 detected sources at S-band is 0.13 Jy, and the flux density of 3209 sources detected at X-band is 0.09 Jy. We find that $70\%$ of detected sources in our campaign are considered compact at X-band and ideal for use in the ICRF and $89\%$ of the 2615 sources detected at both frequencies have a flat spectral index, $\alpha>0.5$